There is a need in the photographic industry to remove substances from processed material to ensure image stability. In deep-tank replenished processing systems material passes from one tank to the next. For example, with respect to processing of color paper, the material passes through a first tank for development, a second tank for bleach/fix and then into a series of wash or stabilizer tanks. The wash tanks are usually inter-connected so that clean washing solution is added to the last of the tank series and the over-flow from the last tank is transferred to the previous tank and so on. In this way the flow of solution is in a direction which is counter to the direction of transport of the paper. This so-called counter-current flow technique enables efficient washing since when the material has the highest content of substances to be removed, the wash solution also has the highest concentration of removed substances and clean solution is only used in the last step when the processed material contains little removable contaminants.
The table below is derived from a mathematical model which predicts the fraction of contaminants remaining in color paper after a four-tank counter-current wash stage in which 194 ml/m2 of solution is added to the last tank. High agitation is assumed which allows equilibrium between substances in the solution and processed material to be rapidly established.
Counter-Current Multiple WashNumber of tanks44Fraction of material left0.000670.00062Total Volume (ml per m2)194776Total time @ 22.5 sec per tank9090
The technique of counter-current washing is widely if not universally adopted with small, so-called Minilab or Microlab equipment and is often also used in large-scale wholesale equipment. More efficient washing can be achieved if more tanks are used in a counter-current series. However, the tanks are bulky and require pumps to provide adequate re-circulation and agitation. Each additional tank incurs additional cost and maintenance.
Shorter washing times can be achieved if the time in each tank is reduced below that required for the material in the coating to be in equilibrium with the material in solution. This can be achieved without undue loss of washing efficiency. For example U.S. Pat. No. 6,106,169 describes a multi-tank unit in which all but the last tank is insufficiently long to provide an immersion time sufficient to reach equilibrium. This unit was found to produce good results with a seven tank configuration giving a total wash-stage time of 20 seconds using as little as 9 ml/m2 of solution.
By reducing the tank volumes, shaping them appropriately and allowing the paper to be transported with the coated side against the curved surface of the interior of the tanks, the agitation/re-circulation pumps could be avoided. However this arrangement required the provision of seven tanks with six cross-over devices to pass the paper from one tank to the next. Such cross-over devices, usually a set of at least two rollers, are expensive and require cleaning and maintenance.
An alternative approach to using curved surfaces in the above multi-stage unit is to use substantially planar, inclined surfaces. The so-called “Inclined Ramp” washing system, see EP M908767, provided a single plane at a 45° angle, to guide the paper in an upward direction with the coated side of the paper against the plane surface. Cleaning/washing solution was added to the top of the inclined plane and ran under gravity under the paper. This surface was not smooth but was textured to provide some agitation. Theoretically this provides a very large number of tanks in a way which is analogous with the theoretical plates of a distillation column. Although each “tank” provides inefficient washing due to the short residence time, the opportunity for material exchange between the paper and increasingly clean water is continuously available. In practice, the efficiency of this device was not high. This was possibly due to the ability of the wash solution to find pathways under the paper surface which allowed rapid descent of a substantial part of the solution. Also in this type of arrangement, it is possible for the paper to drag water from the lower end of the ramp where the solution contains high concentrations of extracted substances to the upper parts, thus contaminating the relatively clean solution flowing down the surface. A number of these problems have been solved, see co-pending application ref 84588, having the same filing date as this application. The problems were solved by providing a planar surface which differs from a truly smooth continuous planar surface in a way which provides a means of controlling the descent of wash solution down the inclined plane so that the descent time of the wash solution is substantially longer than the descent time of the same solution on a smooth planar surface. The preferred rate of descent of the solution is between 0.05 and 10 times the rate of ascent of the paper.
An apparatus based on this principle provides excellent washing which is both rapid and which makes efficient use of wash solution. However some difficulties were encountered.
The paper must be held against the planar surface over its whole area and must be transported up the plane, preferably by simple mechanical means. The paper is often provided in the form of sections of a continuous web of material which is usually manufactured in roll form. The sheets or webs of paper therefore do not naturally lie flat and are subject to strains introduced while in roll form. There is a tendency for the paper to curl and this tends to lift the paper from the planar surface at all four edges.
A rigid support may be applied to the back of the paper in order to apply a force pressing the paper against the plane. However, it is difficult to ensure even pressure without either precise engineering or complicated adjustable locating means whilst also ensuring that the paper can be transported against the resistance provided by the frictional forces produced by the applied pressure. Also, a smooth surface, particularly when wet, provides excessive drag.
It is also important to clean the back of the paper. Contaminants from the processing baths before the wash stage will be present in a film of liquid on the back of the paper. These need to be removed. In addition, contaminants will be transferred to the surface of the means of applying pressure to the back of the paper. The motion of the paper up the plane tends to drag the dirty liquid up the plane. When a length of paper has passed through the wash stage, the surface of the means of applying pressure will tend to contact the washing surface of the inclined plane and contamination of the clean areas of the plane occurs.
The problem to be solved is the application of pressure to the back of the paper in such a way as to ensure contact of the coated surface of the paper with the washing surface over the entire area of the paper whilst allowing the frictional resistance to paper transport to remain sufficiently low as to allow transport to be achieved with the minimal use of paper drive mechanisms, such as rollers or belts. Additionally, the back of the paper and the means of applying pressure must be cleaned in a way which prevents the washing surface being contaminated by waste material transferred from the means of applying pressure when paper is no longer present.